In the prior art, it is known to produce aluminum in coil form from a continuous casting apparatus wherein molten aluminum is delivered from a tundish and cast in the form of a metal sheet or strip and rolled into a coil on a coiler. Generally, in this process, molten aluminum is deposited on a moving chill surface from a tundish having an open outlet. An inlet is provided for the flow of molten metal into the tundish from a source of molten metal. The direct casting of the molten aluminum metal onto a chill wheel, preferably a grooved chill wheel, produces a cast aluminum product at a rapid rate. The aluminum cast strip is wound on a coiler in heated form, generally at a temperature in the range of about 400.degree.-1000.degree. F.
Drag casting apparatus and methods of this type are described, for example, in U.S. Pat. Nos. 4,828,012, 4,896,715, 4,934,443, 4,945,974, 4,940,077 and 4,955,429. The disclosures of these patents are hereby specifically incorporated by reference with respect to the method and apparatus for the production of aluminum strip and coil from molten aluminum or aluminum alloys.
With reference now to FIG. 1, a continuous casting apparatus is illustrated which is typical of prior art continuous casting apparatus using a driven chill surface. The continuous casting apparatus is generally designated by the reference numeral 10 and is seen to include a tundish 1 positioned adjacent a driven chill surface 3. The chill surface 3 comprises the external cylindrical surface of a casting wheel 5. The casting wheel 5 is internally cooled with circulating water or other conventional cooling fluids to extract heat through the chill surface 3 so as to solidify molten metal 7 exiting the tundish 1. A rotary brush 18 contacts the chill surface 3 to remove debris and other impurities prior to casting.
The casting wheel 5 is supported by journal bearings 9 for rotation about a fixed horizontal axis. The journal bearings 9 are supported on the supporting frame 11 which supports both the bearings and the tundish 1. The casting wheel may be driven by a suitable drive means such as a variable speed motor and reduction gear mechanism, not shown, and a drive chain or belt 13 engaging the casting wheel 5.
After the molten metal 7 contacts the chill surface 3 and is solidified as a strip 15, a coiling apparatus 17 accumulates the strip in coil form for further processing. The coiling apparatus may include rollers 23 to guide the cast strip to the coiler.
The continuous casting apparatus 10 may also include a burner 19 to selectively apply heat to the chill surface at a location beneath the tundish 1. In addition, a top roll 21 may be provided which is uncooled or heated, the top roll being mounted for rotation in contact with the molten metal prior to complete solidification of the strip.
However, these types of melt drag casting apparatus present difficulties in casting certain types of alloys. For example, at casting onset, the solidified head of the strip being cast does not release from the casting wheel. Consequently, the cast strip will wrap around the wheel, burning the brushes 18 and damaging the tundish 1. On the other hand, if the cast strip releases too quickly it can become entangled in the cast strip framework or other cast strip components.
Besides alloys having unacceptable release characteristics as described above, certain alloys lack the required physical strength characteristics for start-up conditions. During casting start-up, an alloy must have sufficient stiffness to feed through the strip handling equipment such as the guide rolls 23 located between the casting wheel 5 and the coiler 17. The cast alloy strip must also have sufficient tensile strength so that it is not pulled apart by the feed rolls when wrapping around the coiler 17. In combination with the required strength, the alloy cast strip must also possess sufficient flexibility to take gentle bends and direction changes without breaking.
Highly alloyed materials such as an AA aluminum alloy 5182 exhibit brittleness and high strength which make it difficult to begin casting. On the other hand, high purity aluminum alloys such as AA 1235 do not have sufficient strength to travel from the casting wheel through the material handling components to the coiler 17.
In view of the deficiencies in prior art melt drag casting apparatus, a need has developed to improve the continuous casting start-up sequence so as to successfully continuously cast these types of difficult-to-cast alloys.
In response to this need, the present invention provides a start-up alloying vessel in combination with a melt drag continuous casting apparatus. The start-up alloying vessel permits start-up continuous casting with an alloying composition having the necessary strength and release characteristics to establish steady state casting conditions. Once casting equilibrium is achieved, an alloy composition exhibiting characteristics making it difficult to initiate continuous casting is merged with the start-up alloying composition to produce a cast strip product of the desired composition.
In the prior art, various casting apparatus have been proposed which include an intermediate tundish disposed between a molten metal vessel and a casting surface or mold. In U.S. Pat. No. 3,836,360 to Bray, a launder is disposed between a furnace and a casting mold. A dilute master alloy as a pre-heated wire is fed into the launder while a pure molten copper enters the launder from a furnace. The alloying wire is pre-heated using an electrical circuit prior to submersion into the molten metal in the launder.
U.S. Pat. No. 3,871,870 to Nemoto et al. also discloses the use of an intermediate vessel disposed between a ladle and a mold for alloying purposes. In this patent, rare earth metals are added into the liquid steel in the intermediate vessel. Adding the rare earth elements in the intermediate vessel facilitates desulphurization of the liquid steel.
U.S. Pat. No. 4,010,876 to Steinemann discloses an arrangement for the delivery of predetermined amounts of molten metal from a sealed and heated storage vessel. The arrangement includes an intermediate vessel which permits measurement of the molten charge within the vessel. Inoculation or seeding agents may be added to the molten metal in the intermediate vessel.
U.S. Pat. No. 3,819,365 to McCaulay et al. provides a reaction chamber separate from the casting mold. The reaction chamber is used to add a nodularising agent to the untreated molten metal prior to casting. In U.S. Pat. No. 4,040,468 to Nieman, a wire of added material is inserted into a mixing chamber prior to a casting cavity. The mixing chamber has a semi-circular configuration to enhance mixing action between the wire additive and the molten metal.
U.S. Pat. No. 3,305,902 to Bjorksten uses an intermediate vessel in a casting apparatus to facilitate foaming of the metal. During casting, the molten metal is poured into an intermediate vessel prior to casting into the casting metal. A foaming agent is supplied to the intermediate vessel to foam the molten metal prior to final casting.
The prior art also discloses methods and apparatus concerned with continuous casting start-up. In European Patent Application Number 0127 578 to Buxmann et al., a casting nozzle in a twin-belt continuous casting machine is pre-heated by first casting with molten aluminum followed by casting with grey cast iron. The nozzle is then sufficiently preheated for casting steel. In Japanese Patent Number 63-242450, a method for continuously casting metal strip using two rotating cooling rolls is disclosed which shortens the time from casting start to steady state casting. At first, the tip of a dummy bar for drawing the continuously cast strip is inserted into the gap between the two rotating cooling rolls. An easy melting metal material is added adjacent to the dummy bar. The metal material is easily melted by holding heat of the molten metal as raw material is successively poured from the pouring basin. After casting, the dummy bar containing the easy melting metal is removed.
However, none of the prior art discussed above addresses the problems associated with melt drag casting of metal strip, in particular, aluminum alloys having physical properties causing problems in the start-up of melt drag casting. Further, the prior art does not recognize modifying a select amount of an alloy composition to be cast so as to initiate continuous casting and achieve steady state casting prior to onset of a difficult-to-cast alloy composition.